Conductive coatings made by dispersion of intrinsically conductive polymers (ICPs) such as polyaniline that are significant for technical applications, are described in Synthetic Metals 55-57 (1993) 3780-3785. Such coatings, while offering good clarity, water and heat resistance are limited in the range of surface resistances that they can provide with present formulation technology, typically less than 1.times.10.sup.5 ohm/square. With these formulations it is presently difficult to achieve surface resistances reproducibly within the range 10.sup.5 to 10.sup.10 ohm/square. The limiting factor in controlling the surface resistance arises from the nature of the percolation behavior of the conductive phase, and is common to other two-phase systems of a conductor in an insulating matrix, such as blends of intrinsically conductive polymers, carbon blacks, metal filled polymers, and the like in insulating polymeric matrices. In a two component system wherein a conductive component is dispersed in an insulating matrix, the conductivity of the resulting system remains unchanged until a critical volume fraction of the conductive phase is reached, at which point there is a sudden, very large increase in the bulk conductivity of the system. With further addition of the conductive component there is only marginal change in the conductivity of the system and the conductivity of the system is saturated. The critical volume fraction of conductive filler for the sudden onset of bulk conductivity is referred to as the "percolation threshold." Typically this occurs with a change of conductivity of about 10.sup.-12 to 10.sup.-5 S/cm within a change in volume fraction of the conductive phase of about 0.5 to 3 volume percent. With such a steep increase in conductivity with very small changes in volume fraction of conductive components, it is extremely difficult to reproducibly prepare controlled coatings in the thickness range of about 0.1 to 30 microns that provide the highly desired surface resistances in the electrostatic dissipation (ESD)/antistat range of 10.sup.5 to 10.sup.10 ohm/square. Thus, there is presently a limitation in the art for making coating formulation and blends that reproducibly and controllably provide surface resistances in the 10.sup.5 to 10.sup.10 ohm/square range on insulating substrates.
Blends with intrinsically conductive polymers, especially with dispersible intrinsically conductive polymers in the powdered form are described in U.S. Pat. No. 5,217,649 and PCT/E88/00798. The definitions and concepts described therein are also applicable to the present application and are, therefore, incorporated by reference herein, for such disclosure.
The term "intrinsically conductive polymer" refers to an organic polymer containing a highly conjugated backbone comprising double and triple bonds, aromatic rings, and in some cases heteroatoms such as nitrogen, sulfur, oxygen and the like, which also have been doped with electron donor or electron acceptor dopants to form a charge transfer complex having an electrical conductivity of at least about 1.times.10.sup.-6 S/cm by the four-in-line probe method. Examples of such polymers are polyanilines, polypyrroles, polyacetylenes, polythiophenes, polyphenylenes, and the like.
With the rapid advancement in electronic data processing and communication systems, electronic components have increasingly become smaller and are inherently more sensitive to electrostatic discharge especially during manufacture, handling, and packaging. Thus, the demand for effective static discharge materials has become very acute. Plastics, which are preferred materials for packaging, are insulators with a surface resistance of greater than 10.sup.15 ohm/square and can generate charge by friction that is not effectively dissipated. When such plastics are used for packaging static sensitive materials, static charge buildup can result in an uncontrolled discharge and result in damage to the electronic component that is packaged. Also, uncontrolled discharge can be a safety hazard to the operator in some instances.
For effective controlled discharge protection, the ideal situation is one wherein the electrical charges are dissipated nearly as rapidly as they are generated. Thus, at any point there is no accumulation of charge on the surface of the plastic and, hence, no danger of hazardous electrostatic discharge. In order to do this effectively, the surface resistance of the packaging material should be less than about 1.times.10.sup.10 ohm/square, preferably between 10.sup.5 and 10.sup.10 ohm/square, even under extreme climatic conditions.
Following is a discussion of the various materials for static discharge protection.
1. Antistatic agents such as quaternary ammonium compounds, ethoxylated amines, alkyl sulfonates, and such function by ionic conduction and are dependent on humidity. They typically provide surface resistances in the 10.sup.11 to 10.sup.12 ohm/square range when applied as thin surface coatings, and even under ideal conditions provide only about 10.sup.10 ohm/square. Further, there is a concern about their corrosive effect on electronic components. Thus, they fail to provide the necessary requirements for reliable electrostatic discharge protection.
2. Filled systems such as conductive carbon loaded polymers are capable of offering resistances lower than 10.sup.10 ohm/square, but suffer from the disadvantage that they are black and non-transparent. Further, sloughing is a common problem associated with conductive carbon compounds. Sloughing is the shedding or flaking of carbon particles from the blend upon friction. These flakes are conductive and can cause short circuits in electronic components. Owing to the nature of the common percolation behavior of conductive carbons in insulating polymer matrices, it is extremely difficult to control the surface resistance in the 10 .sup.5 to 10.sup.10 ohm/square range with these compositions.
There recently has been a considerable amount of interest in processing of intrinsically conductive polymers, especially polyaniline. Practical products are being explored.
1. High conductivity coatings and films from polyaniline have been reported in Synthetic Metals 48, (1992) 91, Synthetic Metals 55-57 (1992), 3514-3519, and Applied Physics Letters 60 (1992), 2711. The process involves synthesizing doped polyaniline with hydrochloric acid, neutralizing to obtain the emeraldine base and protonating it again with another acid, in this case preferably camphor sulfonic acid in the presence of m-cresol. Coatings containing pure polyaniline with a high degree of optical transparency and extremely low surface resistances have been prepared. The application of these materials is limited to very few substrates that are resistant to attack by solvent (m-cresol), such as polyethylene terephthalate (PET) and glass. Commonly used plastics in electronic packaging, such as polyvinyl chloride (PVC), polyethylene terephthalate glycol (PETG), and polycarbonate, would be seriously damaged by solvent attack. In addition to the highly complex process by which they are made, a further disadvantage may be seen in the fact that some of the m-cresol remains in the conductive film and potential toxicological problems arise both during the process and later use. Further, it is not known whether the coatings are resistant to heat, water and common cleaning solvents.
2. U.S. Pat. No. 4,526,706 describes a process for preparing conductive latex compositions. The five step process involves dissolving polyaniline base in a water miscible organic solvent, forming a latex by dispersing hydrophobic loadable polymer particles in an aqueous continuous phase, blending the polyaniline and latex solutions, removing the organic solvent and finally, converting the polyaniline base loaded latex to the conductive polyaniline salt loaded latex by acidifying the latex with a suitable acid. The resulting latex can be coated on a polyester substrate with a surface resistance of 10.sup.6 to 10.sup.9 ohm/square. Despite the complicated process, the coating possessed fair transparency, but no data on heat and water/solvent resistance was provided.
In summary, the prior art discloses no simple and effective processes that are available for preparing conductive coatings that are transparent, have good water and heat resistance, and provide controlled surface resistances in the 10.sup.5 to 10.sup.10 ohm/square range that are widely suitable for ESD and antistat applications.
Therefore, there remains a need for conductive coatings that are commercially suitable for electrical and electronic applications, which provide controlled surface resistances in the range 10.sup.5 to 10.sup.10 ohm/square, are substantially transparent and resistant to water and common cleaning solvents such as alcohols, and can be processed by conventional application techniques such as gravure, spray, dip and other coating methods.